The invention relates generally to a magnetic resonance imaging (MRI) system and more particularly to a low eddy current vacuum vessel for enclosing a superconducting magnet in the MRI system and a process for manufacturing the vacuum vessel.
MRI systems utilize superconducting magnets to generate a strong, uniform magnetic field within which a patient or other subject is placed. Magnetic gradient coils and radio-frequency transmit and receive coils then influence gyromagnetic materials in the subject to provoke signals that can be used to form useful images. Other systems that use such coils include spectroscopy systems, magnetic energy storage systems, and superconducting generators.
In use with MRI, a superconducting magnet is disposed in a vacuum vessel that insulates the magnet from the environment during operation. The superconducting magnet also has a coil support structure to support the coil in a cooling mass and a helium vessel for cooling. The helium vessel is a pressure vessel located within the vacuum vessel for thermal isolation and typically contains liquid helium to provide cooling for the superconducting magnet to maintain a temperature of around 4.2 Kelvin for superconducting operation.
The vacuum vessel in an MRI system is generally made of components that are welded together during assembly of the magnet to form a pressure boundary. The function of the vacuum vessel of an MRI magnet is to provide a reliable pressure boundary for maintaining proper vacuum operation. Vacuum vessels known in the art are usually made of metals such as stainless steel, carbon steel, and aluminum. Although, metal vacuum vessels are strong enough to resist vacuum forces, they generate eddy currents and unwanted field distortions in the imaging volume when exposed to an AC field, such as an AC field generated by gradient coils of the MR system. When the magnet is operated in an AC field environment, eddy currents will be induced in those metal components. The eddy currents in the vacuum vessel of a MRI system generate un-wanted field distortions in the imaging volume and adversely affect the image quality. The eddy current heating may also cause structural or thermal problems. That is, the AC losses add to the total heat load and increase costs for maintain the helium at a cryogenic temperature.
While the composition of the vacuum vessel plays a significant role in reducing eddy currents, the structural integrity of the vacuum vessel is also important in order for the MRI system to operate efficiently. The vacuum vessel needs to provide a reliable pressure boundary to maintain vacuum operation. Any leakage and gas permeation into and out of the vacuum vessel will, over time, increase the vacuum pressure and thus increase the heat load of the magnet.
Thus, there is a need for reducing field effect losses from eddy currents in a vacuum vessel and for ensuring that the vacuum vessel is free of leaks to provide a reliable pressure boundary.